I. Field of the Invention
The present invention relates to sheet metal bending brakes.
II. Description of Related Art
There are many previously known portable sheet metal bending brakes that are used in the building construction industry, typically for the installation of aluminum siding. These previously known sheet metal bending brakes typically comprise a frame having a planar work support surface and a plurality of spaced frame members which extend over the work support surface. Each frame member, furthermore, includes a throughbore so that the throughbores in the frame members are aligned with each other.
A plurality of elongated pivot arms are pivotally secured at one end to the frame for movement at their opposite ends toward and away from the work support surface. An elongated clamping bar is typically secured to the other ends of the pivot arms so that the clamp bar extends generally parallel to and spaced apart from the work support surface.
There have been a number of previously known mechanisms for pivotally moving the pivot arms in unison with each other and, accordingly, moving the clamp bar toward and away from the work support surface. Consequently, with the sheet metal positioned on the work support surface, the pivotal movement of the pivot arms toward the work support surface clamps the sheet metal between the clamp bar and the work support surface.
In one previously known sheet metal bending brake described in U.S. Pat. No. 4,671,094 to Break, issued Jun. 9, 1987, a circular opening was formed in each pivot arm so that the circular opening was aligned with, but eccentric to, the throughbores formed through the frame members. An elongated shaft was then rotatably mounted within each frame member throughbore and this shaft extended through the circular openings in the pivot arms.
In order to displace the pivot arms with their attached clamp bar upon rotation of the shaft, a plurality of circular cams were secured to the shaft so that one cam was positioned within the circular opening of each pivot arm. Consequently, pivotal movement of the shaft pivoted each cam due to the coaction of each cam with its opening in the pivot arms which vertically displaced the pivot arms with their attached clamp bar in the desired fashion.
A primary advantage of this previously known “cam lock” bending brake is that the pivot arms are able to lock on different thickness metal only within a relatively small range of thickness variation. Such locking was achieved when the major lobe of each cam was aligned within about 23 degrees or less of vertical when the pivot arms were in a locked position.
Conversely, for sheet metal as well as other materials having a thickness greater than the range accommodated by the bending brake, the cams are rotated outside the range of about 23 degrees with respect to vertical. When this happens, the frictional engagement of the cams with their circular openings in the pivot arms is insufficient to frictionally lock the cams against rotation to the pivot arms. When this occurred, the pivot arms would oftentimes “pop open” in use.